The membrane of choice for extracting hydrogen from high temperature, hydrogen containing gas streams, and for purifying hydrogen to high purity is palladium silver. Typically, the palladium silver membranes are used as tubes, with a wall thickness of about 0.003 inches, a diameter between 1/16th and ⅛th of an inch and a composition between 23% and 25% silver by weight. Palladium silver alloys combine high selectivity, good surface properties, and reasonable mechanical properties. Specifically, palladium silver is ductile, does not embrittle in hydrogen, does not show excessive creep at operating temperatures, and brazes well to stainless steel and other materials of choice. The main problems with palladium silver are its high cost and low hydrogen flux. Practical maximum throughputs with palladium silver are on the order of 1 cubic foot/ft2.minute. These two difficulties combined result in a high cost per flux. Thinner membranes of palladium silver can be used, but it is hard to make thin tubes that are pore-free and even one pore will destroy the selectivity of the membrane.
Thin sheets of palladium copper alloys, or palladium silver alloys have been proposed as alternatives to palladium silver tubes, but currently these sheets do not provide the durability of palladium silver. Palladium copper, in particular can be rolled to a pore free thickness of about 0.001″ (1 mil), and can be made thinner yet using chemical or electrochemical etching. This is about ⅓rd the usable thickness of pore free palladium silver tubes, but flat plate membranes require expensive support structures that block hydrogen flow. The result is that the ratio of system cost to flux is barely less for palladium copper sheets than that for palladium silver tubes. Palladium copper is less ductile than palladium silver and as such cannot be drawn readily into tube shapes.
Palladium coated group 5b metals (V, Nb, and Ta) and alloys are also an alternative to palladium-based membranes, as detailed in U.S. Pat. No. 5,149,420; and I&EC Research 35(1996) 530. These membranes are annealed to remove the oxide film that exists between the coat and the substrate. Similar membranes have also been developed as detailed in U.S. Pat. No. 3,350,846; and U.S. Pat. No. 5,738,708. These membranes either did not include the annealing step, allow for alloys or were produced by costly ion sputtering in vacuum.
The flux with these alloys was much higher than with palladium silver or palladium copper, and the material cost is much lower. Problems associated with group 5b based membranes include embrittlement in hydrogen, and palladium substrate interdiffusion. Several alloys have been tried as substrates to eliminate these problems with group 5b based membranes. Particularly note worthy here are membranes made of vanadium nickel, vanadium nickel cobalt, and vanadium chromium titanium. These alloys embrittle far less than the group 5b metals, they start out more brittle, and as such they are not readily drawn into tubes; their physical properties are rather like palladium-copper, mentioned above. Further, the palladium substrate interdiffusion problem remains, though it seems to decrease in rough proportion to hydrogen solubility in the alloy.
Applicant has come up with a non-limiting theory for the general observation that, the less hydrogen the alloy absorbs, the slower the palladium substrate interdiffusion. This is that hydrogen in the metal stabilizes vacancies in the group 5b metals, increasing their number substantially at any temperature. Since hydrogen permeation generally requires a fairly high hydrogen solubility in the metal (permeability≈solubility×diffusivity) this explanation at first suggested that there would be no high-permeation substrate alloy that would have a low palladium-substrate diffusivity. A high palladium substrate interdiffusion rate requires that membranes must be made with fairly thick palladium coats, and this increases the cost of the membrane. Edlund has suggested that a thin coat could be used despite high interdiffusion if the substrate is coated with a ceramic interlayer, but these layers add cost and decrease the flux. Palladium, or palladium alloys must be applied to all the alloys treated so far because palladium increases the rate of hydrogen uptake and release from the membrane, greatly increasing the overall flux. Vanadium alloys containing 10% Ni or Pd (by weight) had fairly good surface properties, and came near to not needing a palladium coat. It has been suggested that palladium alloys would make better surface coatings as hydrogen is observed to create fewer defects in such alloys than in pure palladium.
The embrittlement problem with hydrogen permeation membrane materials is seen also with hydrogen storage materials. The most common hydrogen storage materials are metallic compounds like FeTi, and LaNi5. These materials are used for long-term hydrogen storage, for hydrogen removal (gettering) and as transitory hydrogen storage in hydride compressors, pumps, and nickel-metal hydride batteries. All the materials in common use are known to embrittle and to crumble with repeated cycling. This causes a variety of problems, particularly in mechanical stress to their containers, and in diminished transport of heat and hydrogen seen in the crumbled alloys.
Gschneidner et al. showed that several B2 rare earth compounds are ductile despite showing ordered CsCl structure. Gschneidner et al, Nature Materials 2(2003)587. These structures are intercalated body center cubic, also known by the cP2 Pearson symbol, or Pm3m designations. Gschneidner et al have shown that YAg was more ductile than a common aluminum alloy. The properties of these compounds as hydrogen permeation or storage materials were not established.
Thus, there exists a need for a new class of low cost allows for hydrogen permeation membranes and applications that overcomes the limitations of the prior art.